Intermittent RCD Trip Problem for Last 6 Years


Postby Trippy2 » Thu Dec 14, 2017 2:23 pm

Where do I start, I have consumer unit with 1 RCD, so when it trips it takes out all power to the house. It has regularly tripped since moving into the house 6 years ago. I had an extension put on 3 years ago and asked the electrician, who quoted £8k to change the consumer unit, due to age of house (1950's) and need to rewire. Discounted that option as could put up with a trip circa once a fortnight and decided to use an old mobile phone plugged in to text me every time it tripped - it has been doing this since Jan 2016.

I now have date and time of the 73 times it has tripped since then, it tends to go more often on the days the washing machine is on, but not all the time, other times it will go when we are on holiday or not at home. The one strange statistic is that the earliest it has tripped is 08:24 and the latest is 23:13, but never overnight, is there any reason for this? Half of the trips have happened between 09:00 and 13:00.

I have had a ramp test and am told the RCD is working fine. Other than that electricians haven't generally been too interested in solving the issue.

So I haven't even isolated the appliance that is causing the trip, but could it be a build up of leakage across several?

I am currently using an RCD plug at the end of each appliance one by one, but presume if the main RCD goes it will take out the plug RCD even if it isn't that appliance causing the issue?

What is my next move?

Thanks for any advice.
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Postby Mr White » Thu Dec 14, 2017 9:47 pm

Trippy2 wrote:What is my next move?


"Cross your fingers"

The problem you have, is that it is an intermittent problem, that is why no one is eager to look at it.

It could also be a you suggest, a combination of small electrical leaks.
Unless you have an insulation resistance meter (And know how to use one) you really will have to unplug several appliances at once, leave them off for a few days and cross you fingers.
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Postby ericmark » Thu Dec 14, 2017 11:02 pm

The RCD comes in many flavours, pre 2008 most whole house RCD's were 100 mA S type after 2008 30 mA became the norm.

So question one is what size RCD have you got fitted?

Also standard for a RCD is to trip between 1/2 and full rated trip current, however there are versions which light up a warning light at 1/2 rated current and don't trip until 90 ~ 100% of tripping current not 50 ~ 100%.

Old RCD's had no electronics, and spikes on the supply could trip them, the newer versions are electronic with built in any spike software to stop them tripping with spikes on the line.

There are even auto resetting RCD's however since 2012 not permitted for domestic and also cost around £300 each.

There is also a device called a RCBO which is a MCB and RCD combined, they are longer than MCB's so will not fit in all consumer units, they are in the main single pole switching which is not suitable for a TT supply, some makes do make double pole switching, but overload only measured on line pole, so question is will a RCBO fit in your consumer unit? And what is your supply TN or TT? (TT means you have an earth rod)

In my house I have two consumer units ready to be fitted, planned before 2004 so does not come under Part P, however the existing fuse boxes are fed from two RCD's in a separate box, so even with old Wilex consumer units you can have RCD protection, would not do it that way now, but when fitted in 1994 not many houses had RCD's.

10 mA RCD's tend to be expensive, around £40 plus the enclosure, however you can get them, there were 10 mA plug in types, but not seen them for years.

There are three tests to find why tripping, the first is testing the RCD, it measures the time taken, must be less than 40 mS and tests 6 times. Twice at 1/2 current (some will ramp test) once positive and once negative all tests are done twice on pos and neg half cycle, at 1/2 it should not trip, and full it must trip but can exceed the 40 mS and at 5 times must trip in the 40 mS time limit. Clearly you can't really do this test the meter is too expensive you need to call some one in to do it.

The next is to test equipment, there are two methods, one is using a clamp on ammeter to measure the leakage, the other is to use an insulation tester which generates 500 volt to measure the meg ohms of the two not sure which is best, my clamp on will not measure down to 30 mA the Kewtech KT200 it states will measure down to 10 mA at just under £50 but not even sure if that will help as you may have a number of items leaking 2 mA which all adds up to over 15 mA which could cause a trip.

For the cheapest insulation tester found one at £60, likely a better option but it will not find anything which needs power to select it, like the heater in a frost free freezer which may only activate after 5 hours use. Some times you can find fault by testing neutral to earth rather than line to earth.

So now you can help me, how did you set up a mobile phone to tell you where there is a power failure? I have an insulation tester but RCD tester is broken, I live in North Wales.
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Postby Trippy2 » Fri Dec 15, 2017 9:34 am

Thanks for your replies Ericmark and Mr White.

I was chatting to someone last night who thought the issue would be with a frost free fridge freezer I have (Beko), as that would explain why it doesn't trip between midnight and 8:30ish. I also think there could be a build up of leakage across all appliance as I have 2 fridge freezers (1 frost free), dishwasher, water softener, combi boiler, tumble dryer and washing machine. I'm sure the consumer unit is post 2008 and has 30mA RCD.

Now for the mobile phone, I got an old android phone from work and found a free power outage app on google play. Apple does not do any such app. I then bought a £10 pay as you go sim card from Tesco mobile as this is the only company that does pure pay as you go - most others just do monthly contracts and I didn't want to spend £10 per month for 1 or 2 texts! I've never had to top up the phone after receiving 73 texts. Just make sure phone is constantly plugged in.
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Postby ericmark » Sat Dec 16, 2017 12:06 am

Thanks I will look into that, have a spare phone and Tesco sim so just need to find app and set up.
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